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Journal of Experimental and Theoretical Physics

Distribution function and diffusion of electrons in the Rydberg energy space in a nonideal ultracold plasma

Sergey Ya. Bronin, B. B. Zeener, B. V. Zelener, Edward A. Manykin, Danila Khikhlukha

Published Apr 1, 2011• 9 citations
Publisher

Abstract

The distribution functions and diffusion coefficient of electrons in the Rydberg energy space in an ultracold plasma are obtained as functions of temperature by numerically solving a system of kinetic balance equations. As the initial conditions, the results obtained in previous papers by the molecular dynamics method are used. From calculation of Rydberg electron fluxes, the temperature dependence of the recombination coefficient is obtained, in good agreement with the recombination coefficient calculated previously by the molecular dynamics method.

DOI
10.1134/s1063776111030034

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The distribution functions and diffusion coefficient of electrons in the Rydberg energy space in an ultracold plasma are obtained as functions of temperature by numerically solving a system of kinetic balance equations. As the initial conditions, the results obtained in previous papers by the molecular dynamics method are used. From calculation of Rydberg electron fluxes, the temperature dependence of the recombination coefficient is obtained, in good agreement with the recombination coefficient calculated previously by the molecular dynamics method.
The electron distribution function and diffusion coefficient in energy space have been calculated for the first time for a weakly coupled ultracold plasma in a magnetic field in the range of magnetic fields B = 100−50000 G for various temperatures. The dependence of these characteristics on the magnetic field is analyzed and the distribution function is shown to depend on the electron energy shift in a magnetic field. The position of the “bottleneck” of the distribution function has been found to be shifted toward negative energies with increasing magnetic field. The electron velocity autocorrelators as a function of the magnetic field have been calculated; their behavior suggests that the frequency of collisions between charged particles decreases significantly with increasing magnetic field. The collisional recombination coefficient α B has been calculated in the diffusion approximation for a weakly coupled ultracold plasma in a magnetic field. An increase in magnetic field is shown to lead to a decrease in α B and this decrease can be several orders of magnitude.
Theoretical investigations of the properties of Rydberg matter, more precisely, a nonideal ultracold plasma, which is one of the forms of this matter, are reviewed. Monte Carlo and molecular dynamics studies indicate that structures with both short- and long-range orders can be formed and that the recombination coefficient in the case of strong nonideality, can be several orders of magnitude smaller than that under ideal conditions. These and other results obtained in the past several years are discussed.
A kinetic model is developed to describe ultracold nonideal Rydberg plasmas, which allows all stages of the generation and decay of such a plasma to be sequentially traced. The plasma kinetics is considered on the basis of available experimental data corresponding to a nonideality parameter of γ ∼ 1. The results of theoretical analysis are in good agreement with experiment. Calculations show evidence of a significantly decreased recombination rate and, hence, of the possible formation of a metastable structure in the plasma under consideration. The distribution of the number of excited atoms is determined for the plasma with N e = N i = 7 × 10 5 and E e = 9 K. The observed behavior of the number and density of particles as functions of the time and principal quantum number is explained. It is suggested that the distribution of excited atoms for the given parameters has a maximum for the state with k = 25.
Double-resonant photoexcitation of nitric oxide in a molecular beam creates a dense ensemble of 50f(2) Rydberg states, which evolves to form a plasma of free electrons trapped in the potential well of an NO + space charge. The plasma travels at the velocity of the molecular beam and, on passing through a grounded grid, yields an electron time-of-flight signal that gauges the plasma size and quantity of trapped electrons. This plasma expands at a rate that fits with an electron temperature as low as 5 K, colder than typically observed for atomic ultracold plasmas. The recombination of molecular NO + cations with electrons forms neutral molecules excited by more than twice the energy of the NO chemical bond, and the question arises whether neutral fragmentation plays a role in shaping the redistribution of energy and particle density that directs the short-time evolution from Rydberg gas to plasma. To explore this question, we adapt a coupled rate-equations model established for atomic ultracold plasmas to describe the energy-grained avalanche of electron–Rydberg and electron–ion collisions in our system. Adding channels of Rydberg predissociation and two-body, electron–cation dissociative recombination to the atomic formalism, we investigate the kinetics by which this relaxation distributes particle density and energy over Rydberg states, free electrons and neutral fragments. The results of this investigation point to conditions under which such processes can effect the steady-state temperature of plasma electrons.
We present a kinetic model based on coupled rate-equation simulations that accounts for state-detailed rate processes in the evolution of a molecular Rydberg gas to plasma. Calculations support a steady-state picture of Rydberg predissociation and its contribution to plasma dissipation. Inelastic collisions of electrons with Rydberg states efficiently transfer energy from and to the thermal bath of free electrons, giving rise to quasi equilibria of relatively high temperature. Dissipative effects that remove low- n population change the time course of this heating. Including statistically sampled channels of Rydberg predissociation with n - and ℓ-dependent lifetimes slows the relaxation to quasi equilibrium and retards the initial electron temperature increase. At later times, the dissociative loss of low- n population displaces this quasi equilibrium leading to continuous electron heating. Dissociation products appear at a rate that is controlled initially by the rate of Rydberg relaxation to quasi equilibrium. Thereafter, the overall rate at which the total dissociation consumes neutral molecules determines the decay rate for all levels. Simulations predict a total predissociation rate that is smaller than the initial rate of direct electron–ion dissociative recombination. At later times, residual predissociation displays an apparent first-order decay with a rate constant that fits well with experimental observations.